Photographic light-sensitive materials prepared by coating silver halide emulsions generally suffer various mechanical stresses. For example, negative films for ordinary use are folded upon winding into the cassette in which they are sold or loading in a camera, or are stretched upon photographing.
On the other hand, sheet-like films such as light-sensitive materials for printing or X-ray sensitive materials for direct medical use are often folded or bent since they are directly handled.
Every light-sensitive material suffers great stress upon cutting or finishing.
When various stresses as described above are imposed on photographic light-sensitive materials, such stresses reach silver halide grains present through the gelatin medium which is a binder for the silver halide grains or through a plastic film support. Stretched silver halide grains are known to cause changes in photographic properties of photographic light-sensitive materials, as described in , for example, K. B. Mather, J. Opt. Soc. Am., 38, 1054 (1948), P. Faelens and P. de Smet, Sci. et Ind. Phot., 25, No. 5, 178 (1954), P. Faelens, J. Phot. Sci., 2, 105 (1954), etc.
Desensitization of areas where stress has been imposed causes sensitization or fogging, which seriously deteriorates image quality and, with X-ray sensitive materials, can lead to an incorrect diagnosis.
Therefore, it has been desired in the art to develop photographic light-sensitive materials which suffer no harmful influence on photographic properties when they are subjected to such stresses.
As techniques for improving stress properties, it is known to prevent stress from reaching grains by incorporating a polymer or a plasticizer, such as an emulsion, or by reducing the ratio of silver halide to gelatin in a silver halide emulsion.
For example, British Pat. No. 738,618 discloses using heterocyclic compounds, British Pat. No. 738,637 discloses using alkyl phthalates, British Pat. No. 738,639 discloses using alkyl esters, U.S. Pat. No. 2,960,404 discloses using polyhydric alcohols, U.S. Pat. No. 3,121,060 discloses using carboxyalkyl celluloses, Japanese Patent Application (OPI) No. 5017/74 discloses using paraffins and carboxylates (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"), and Japanese Patent Publication No. 28086/78 discloses using alkyl acrylates and organic acids.
However, since the addition of a plasticizer decreases the mechanical strength of an emulsion layer, the amount of the plasticizer to be added is limited. On the other hand, the use of increased gelatin delays development processing speed. Thus, these two techniques provide sufficient effects only with difficulty. Therefore, it would be most desirable to be able to prepare silver halide grains which themselves resist stresses.
When silver nitrate is added to a solution containing, for example, gelatin, KBr, and KI, there results an emulsion which undergoes a serious reduction in light sensitivity when stress is imposed thereto. This reduction in light sensitivity is quite inconvenient, and is removed with pure silver bromide grains or completely uniform silver bromoiodide grains formed by adding a silver nitrate solution and a halide solution according to a double jet-mixing method while avoiding the reproduction of nuclei. However, such grains are easily fogged upon being stressed, which is undesirable.
On the other hand, Japanese Patent Application (OPI) No. 22408/78, Japanese Patent Publication No. 13162/68, J. Photo. Sci., 24, 198 (1976), etc., describe enhancing development activity or enhancing light sensitivity using layered-structure silver halide grains comprising a core with a plurality of shells.
However, silver halide grains obtained for these purposes are not necessarily improved with respect to stress properties, and have problems with desensitization, fogging, etc., upon being stressed. For example, layer structure silver halide grains comprising pure silver bromide (core)/silver bromoiodide (iodide content: 1 mol%)/pure silver bromide as described in Japanese Patent Application (OPI) No. 22408/78 undergo strong fogging when stressed, thus having problems with stress properties similar to conventional uniform silver bromoiodide emulsions.
Silver halide grains having a coating layer formed by substituting the halide in the outermost layer are described in West German Pat. No. 2,932,650, Japanese Patent Application (OPI) Nos. 2417/76, 17436/76, 11927/77, etc. However, they cannot be practically used as negative emulsions since they cause development inhibition and fail to provide sufficient sensitivity, though they accelerate fixing speed.
Positive (internal latent image-forming) silver halide grains comprising an internal core having a plurality of coating layers formed by substitution of halide are known and described in detail in U.S. Pat. Nos. 2,592,250 and 4,075,020, Japanese Patent Application (OPI) No. 127549/80, etc. These silver halide grains are often used in internal latent image-forming, direct positive light-sensitive materials for a diffusion transfer process or the like. They naturally possess too high an internal sensitivity for negative working emulsions to which the present invention is directed.
West German Pat. No. 2,932,650 describes sensitizing the surface of this type of silver halide grains, but such silver halide emulsions do not show improved stress properties.
For example, foregoing Japanese Patent Application (OPI) No. 127549/80 describes an emulsion of silver halide grains prepared by substituting bromide for chloride of the core and iodide for bromide of the core to prepare almost 100% silver iodide cores, then coating them with silver bromoiodide. However, this emulsion strongly undergoes stress desensitization, thus not being practically usable. Even when the grain surfaces are sensitized to convert them to the negative working type, strong stress desensitization still takes place so that the emulsion cannot be practically used.